Begin by putting your beef and liver in the freezer for 30 minutes - this makes them much easier to slice super thin.
While waiting, cut your onions into rings and set them aside.
Take out your semi-frozen meat and slice it as thin as you possibly can, like paper. If you find this tricky, your local butcher can do this for you.
Put all your sliced meat in a bowl. Add the soy sauce, salt, and black pepper. Mix everything well so all the meat is coated. Cover and let it sit in the fridge for at least one hour.
Set up your cooking station. Put a wire rack or strainer over a tray - this is where your cooked meat will drain. Fill your wok or deep pan with enough oil for deep frying, about 2 inches deep. Heat the oil until it's very hot (375°F/190°C). Test it by dropping a tiny piece of meat - it should sizzle right away.
Now for the frying: Add small batches of meat to the hot oil. Don't put too much at once or it won't cook properly. Fry until the edges get crispy and start to curl up, about 2-3 minutes. Using a strainer or slotted spoon, take out the meat and put it on your draining rack.
Here's the final touch: While the meat is still really hot, quickly mix it with your fresh onion rings. The heat from the meat will slightly warm the onions but keep their crunch.
Serve right away while everything's hot and crispy. If you're making the traditional dipping sauce, just mix some vinegar, soy sauce, and chopped chilies in a small bowl.
Remember to cook your beef and liver separately since liver cooks faster than beef. And if you're cooking for a crowd, you can fry the meat ahead of time and just give it a quick refry right before serving to make it crispy again.